Repeaters are used today in two-way optical line links for fault tracing, and these repeaters amplify optical signals electrically. The present repeaters therefore include an opto-electric converter for converting the optical signal to an electrical signal, an electronic amplifier which amplifies the electrical signal and an electro-optic converter which reconverts the electrical signal to an optical signal. An example of such repeater is disclosed in U.S. Pat. No. 4,300,239 to Wakabayashi et al. Fault tracing in optical links in accordance with this patent is performed by continuous monitoring, an end terminal transmits a test signal which points out a repeater for testing. The test signal is amplified in the selected repeater and an optical switch is subsequently placed in an ON condition, a small part of the test signal then being led through the switch and returned to that end terminal which sent out the test signal. In this end terminal the transmitted test signal is compared with the returned test signal for determining whether there is any fault in the selected repeater. All repeaters in the link are checked in this way. In a selected repeater the switch is thus placed in the ON condition, only a small portion of the test signal being run through the switch. The test signal is therefore amplified once again before it leaves the selected repeater. For a repeater which has not been pointed out, the optical switch is at OFF condition and the test signal is run through the repeater without passing through the switch.